1. Field of the Invention
The present invention relates to a magnetic field measuring apparatus for measuring a magnetic field generated by a current flowing in one of a plurality of parallel wires without being affected by magnetic fields generated by adjacent wires, and an apparatus for measuring the spatial resolution of a magnetic field detector.
2. Description of the Related Art
Heretofore, magnetic field detectors for measuring currents flowing in wires on printed-circuit boards, etc. have typically been of a shielded-loop design having a formed semirigid coaxial line as disclosed in "Measurement of Near Fields of Antennas and Scatters" (IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. AP-21, NO. 4, JULY 1973, pp 446-460, for example. According to a magnetic field measuring process effected by the shielded-loop magnetic field detector, it is general practice to fix the magnetic field detector directly above a wire and determine a current flowing in the wire from the measured magnetic field based on the Ampere's rule. Actually, the magnetic field detector detects a voltage depending on the magnetic field, and hence it is necessary to determine in advance a coefficient for converting an output voltage of the magnetic field detector into a corresponding magnetic field, as disclosed in "Time Domain Magnetic Field Waveform Measurement near Printed Circuit Boards", (T. IEE Japan, Vol. 117-A, NO. 5, May 1997, pp 523-530).
FIG. 1 of the accompanying drawings shows such a conventional shielded-loop magnetic field detector. As shown in FIG. 1, a looped coaxial line 101 has terminal ends 102 short-circuited to each other or terminated with a non-reflective terminator. The looped coaxial line 101 has a lowermost end to be placed most closely to a wire to be measured, the lowermost end having a gap 104 leaving a central conductor 103. The central conductor 103 and an outer conductor are short-circuited to each other at a terminal portion 105 of the gap 104. The magnetic field detector produces an output signal commensurate with a magnetic flux that runs across a loop plane 106. If the magnetic field detector is to measure a magnetic field produced by one of a plurality of parallel wires, it is necessary to reduce the size of the loop plane 106 and bring the loop plane closely to the wire to be measured in order to increase the spatial resolution of the magnetic field detector.
FIG. 2 of the accompanying drawings shows a measuring sequence of the shielded-loop magnetic field detector shown in FIG. 1. According to the measuring sequence, the magnetic field detector fixed by a scanning device is positioned directly above the wire to be measured and brought as closely to the wire as possible, and measures a magnetic field in the vicinity of the wire.
If there are no any other wires on the printed-circuit board than the wire to be measured or any other wires on the printed-circuit board are sufficiently spaced from the wire to be measured, then the magnetic field detector can measure the magnetic field produced by the wire without concern over any special considerations. However, if there is an adjacent wire closely to the wire to be measured, then the magnetic field detector detects a combination of magnetic fields produced by both the wires. Therefore, for measuring only the magnetic field produced by the wire to be measured, it is necessary to minimize the magnetic field detector as much as possible and bring the magnetic field detector as closely to the wire to be measured as possible. Since, however, the magnetic field generated due to the current flowing in the wire has a certain spatial distribution, there is a certain limitation on efforts to suppress the effect of the adjacent wire if only the above requirements are met.